Embodiments of the invention relate generally to diagnostic imaging and, more particularly, to a method and apparatus for reduction of metal artifacts in CT images.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan or cone-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
Generally, in the absence of object scatter, a system derives behavior at a different energy based on a signal from two relative regions of photon energy from the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. In a given energy region relevant to medical CT, two physical processes dominate the x-ray attenuation: (1) Compton scatter and the (2) photoelectric effect. The detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged. Furthermore, detected signals from the two energy regions provide sufficient information to determine the relative composition of an object composed of two hypothetical materials, or the effective atomic number distribution with the scanned object.
Techniques to obtain energy sensitive measurements comprise: (1) scan with two distinctive energy spectra and (2) detect photon energy according to energy deposition in the detector. Such measurements provide energy discrimination and material characterization, and may be used to generate reconstructed images using a base material decomposition (BMD) algorithm. A conventional BMD algorithm is based on the concept that, in an energy region for medical CT, the x-ray attenuation of any given material can be represented by a proper density mix of two materials with distinct x-ray attenuation properties, referred to as the base or basis materials. The BMD algorithm computes two CT images that represent the equivalent density of one of the base materials based on the measured projections at high and low x-ray photon energy spectra, respectively.
A principle objective of energy sensitive scanning is to obtain diagnostic CT images that enhance information (contrast separation, material specificity, etc.) within the image by utilizing two or more scans at different chromatic energy states. A number of techniques have been proposed to achieve energy sensitive scanning including acquiring two or more scans either (1) back-to-back sequentially in time where the scans require multiple rotations of the gantry around the subject or (2) interleaved as a function of the rotation angle requiring one rotation around the subject, in which the tube operates at, for instance, 80 kVp and 140 kVp potentials.
High frequency generators have made it possible to switch the kVp potential of the high frequency electromagnetic energy projection source on alternating views. As a result, data for two or more energy sensitive scans may be obtained in a temporally interleaved fashion rather than two separate scans made several seconds apart as typically occurs with previous CT technology. The interleaved projection data may furthermore be registered so that the same path lengths are defined at each energy using, for example, some form of interpolation.
However, it is known that objects with high x-ray absorption properties (e.g., metal) can cause artifacts in reconstructed CT images, often resulting in images having low- or non-diagnostic image quality. For example, metal implants such as amalgam dental fillings, joint replacements (i.e., plates and/or pins used in hips, knees, shoulders, etc.), surgical clips, or other hardware may generate streak or starburst artifacts in the formation of such images. Such artifacts typically result from the sharp difference in signal attenuation at the boundary of the metal implants and a patient's anatomy.
Many correction techniques or methods are known for reducing or altering such artifact streaks. For example, one known technique for reducing artifact streaks includes the use of iterative image reconstruction algorithms with weighting designed to ameliorate the metal artifacts. However, full iterative reconstruction is not widely used clinically.
Another technique for correcting metal artifacts includes performing “projection completion” whereby a corrupted portion of a projection is “completed” or replaced with synthetic projection data having more favorable properties. A shortcoming of projection completion methods is robustness in terms of differing reconstruction parameters (e.g., fields of view), acquisition protocols (e.g., axial or helical), method of metal segmentation and projection interpolation, metal replacement, and the like. Furthermore, projection completion techniques are not directly applicable to multi-energy imaging.
Therefore, it would be desirable to develop an apparatus and method to reduce metal artifacts that provides consistent results using non-iterative reconstruction frameworks. Also, that addresses the above-described short-comings of projection completion methods. Furthermore, it would be desirable to design an apparatus and method for reducing metal artifacts in CT images applicable to multi-energy imaging.